In the drilling of boreholes such as oil or gas wells, a drilling fluid (hereinafter referred to as “drilling mud”) is circulated through the well during drilling in order to, inter alia, remove drilled cuttings, balance the pressure of formation fluids to prevent influxes and maintain the stability of the borehole. In order to provide the required density and viscosity, the mud can include certain solids such as barite and bentonite as well as solids derived from the drilling action. When drilling oil and gas wells, it is quite common to encounter subterranean formations which are porous. If the hydrostatic pressure of the drilling fluid is greater than the pressure of fluids in such formations, mud will penetrate the formation. Generally the pore size of such formations is sufficient to admit the liquid components and very fine solids but to filter out the other solids such as barite or bentonite. These filtered solids form on the borehole wall as a filtercake. This filtercake grows and compacts until the differential pressure is balanced by the stress in the filtercake.
The filtercake has a much lower permeability than the formation. Filtercake can therefore be useful as it acts as a barrier to prevent fluid loss from the drilling mud to the formation. A number of was of forming filter cakes are known in the art, including the use of bridging particles, cuttings created by the drilling process, polymeric additives, and precipitates. Filtercakes, therefore, have different compositional structures, based on the additives, base fluid, and cuttings.
If a filtercake becomes too thick, problems can occur. For example, filtercakes can be a significant contributor to differential sticking: if the drill string or bottomhole assembly (BHA) is allowed to rest against the wellbore wall, the filtercake can continue to grow around the contact point, and must be yielded in order to free the BHA. Emphasis has therefore been placed on developing thin, highly impermeable but weak filtercakes, which minimize their contribution to severity of differential sticking, and also ease clean-up in the reservoir section.
During the drilling of a borehole through underground formations, the drill string assembly undergoes considerable sliding contact with both the steel casing and rock formations. This sliding contact results primarily from the rotational and axial movements of the drill string assembly in the borehole. Friction between the moving surface of the drill string assembly and the stationary surfaces of the casing and formation creates considerable drag on the drill string and results in excessive torque and drag during drilling operations. The problem caused by friction is inherent in any drilling operation, but it is especially troublesome in directionally drilled wells or extended reach drilling (ERD) wells. Directional drilling or ERD is the intentional deviation of a wellbore from the vertical. In some cases the angle from the vertical may be as great as ninety degrees from the vertical. Such wells are commonly referred to as horizontal wells and may be drilled to a considerable depth and considerable distance from the drilling platform. Friction between the drill string assembly and the wellbore walls is also observed when the drill bit (and assembly) is pulled out of hole.
In all drilling operations, the drill string assembly has a tendency to rest against the side of the borehole or the well casing, but this tendency is much greater in directionally drilled wells because of the effect of gravity. As the drill string increases in length or degree of vertical deflection, the amount of friction created by the rotating drill string assembly also increases. To overcome this increase in friction, additional power is required to rotate the drill string assembly. In some cases, the friction between the drill string assembly and the casing wall or borehole exceeds the maximum torque that can be tolerated by the drill string assembly and/or maximum torque capacity of the drill rig and drilling operations must cease. Consequently, the depth to which wells can be drilled using available directional drilling equipment and techniques is limited.
One method for reducing the friction caused by the contact between the drill string assembly and casing (in case of a cased hole) or borehole (in case of an open hole) is improving the lubricity of drilling muds. In industry drilling operations, attempts have been made to reduce friction through, mainly, using water and/or oil based mud solutions containing various types of expensive and often environmentally unfriendly additives. Diesel and other mineral oils are also often used as lubricants, but there is a problem with the disposal of the mud. Certain minerals such as bentonite are known to help reduce friction between the drill string assembly and an open borehole. Materials such as TEFLON® have been used to reduce friction, however these lack durability and strength. Other additives include vegetable oils, asphalt, graphite, detergents and walnut hulls, but each has its own limitations.